“I think rivers are treated as a renewable resource when they’re really not,” John Bolten, a hydrologist and the associate program manager of water resources at NASA’s Applied Sciences Program, told Business Insider. “It’s remarkable how many people are dependent on access to clean water. If rivers are not conserved and used properly, it’s a detriment to everyone.”

That’s well worth considering on World Rivers Day, held annually at the end of September.

To keep an eye on the planet and its most precious resource, Bolten and other researchers study image data from advanced satellites that orbit Earth.

“Using satellite observations, we’re able to have a regional perspective, and to have a global perspective, on water,” Bolten said. He added that – thanks to decades’ worth of observations – those perspectives also span across time.

Here are some of the most telling satellite images of rivers (which we sourced primarily from NASA Earth Observatory) and what they reveal about our close and often contentious relationship with vital waterways.

North Carolina is still dealing with the aftermath of Hurricane Florence. Here’s the Trent River, near Trenton, in July 2017 — long before the catastrophic storm dumped eight trillion gallons of water on the state.

All of that water had to go somewhere. By September 17, 2018, the Trent River crested at nearly 30 feet — double the height where floods for the river begin. Nearby farms, homes, and businesses were inundated, as this picture from September 19 shows.

In other parts of the world, such as Kazakhstan and Uzbekistan, the problem is too little water instead of too much. In the 1960s, the Soviet Union diverted the Syr Darya and the Amu Darya rivers to help grow food. But over time the Aral Sea shrank.

“They changed the natural flow of that system, and they changed the entire dynamic for agriculture,” Bolten said. “But it was an environmental disaster.” Decades of runoff sediments from farming, including fertilisers and pesticides, were exposed to the air and blown around as a hazardous dust.

Efforts to recover and preserve some of the Aral Sea’s water have mostly floundered, and the ramifications are expansive and ongoing. “The loss of the moderating influence of such a large body of water made winters colder and summers hotter and drier,” NASA says.

It’s harnessed and controlled by a series of dams and reservoirs. Shown here is the Hoover Dam, which provides power to Arizona, California, and Nevada.

“The Colorado used to be a raging river when it met Mexico. It’s a trickle now. It’s becoming an ephemeral stream,” Bolten said. The situation has become apparent at Glen Canyon Dam in Utah, which is also on the Colorado River.

Heavy use of Lake Powell’s water for irrigation, along with decreasing snowpack and rainfall, has heavily depleted the reservoir, and NASA does not expect the situation to improve with climate change. “Precipitation declines combined with booming urban populations will present a significant challenge to Western water managers in the near future,” NASA said.

Human dependence on river systems is on full display in Egypt’s Nile River Valley and Delta (shown here at night in 2012), which NASA says comprises just 5% of the country’s land — but is where 97% of its population lives. “Very few rivers flow just in one country, though,” Bolten said.

Bolten added: “There’s often political tension, and that leads to difficulties in sharing data and properly managing these resources.” The Grand Ethiopian Renaissance Dam project is one such example. While the project stands to benefit Ethiopia, it could affect populations thousands of miles downstream.

Humans have struggled to deal with the power and influence of rivers in other ways. In China, the Huang He (or Yellow River) fills with sediments and dumps them into the Yellow Sea. Here’s the mouth of the river in 1989.

NASA says the lack of flow has damaged wetlands that can curb pollution and protect against flooding, and it has caused sediments to build up on the river bottom, further raising the risk of floods.

In 2016, photos of a blood-red Daldykan River in Norilsk, Russia, spread across the internet. NASA says satellite imagery reveals this was not an unprecedented sight — a similar scene happened in 2001. Pollution from a metal-extracting industrial complex is believed to be the cause.

Sometimes what can’t be seen is most revealing. This satellite image shows the city of New Orleans just south of Lake Pontchartrain. The black feature running through the middle is the Mississippi River, and colours show subsidence: where parts of the city are rising (purple) and sinking (green).

River-altering structures like the Oroville Dam in California can create game-changing water and energy resources. But they aren’t without risks: structural failures can put downstream communities in harm’s way. In fact, accounting for all deaths per unit of electricity generated, hydroelectric dams are one of the most risky power sources.

One of Bolten’s research areas is the Mekong River system in Southeast Asia. He says about 60 million people in five countries depend on the river basin, making it “one of the most intensely managed rivers in the world.” Shown here is the Lower Sesan II dam in Cambodia in February 2017.

The reservoir is on the Sesan River and Srepok River, and the dam is expected to bring 400 megawatts of electricity to the region. But the reservoir is displacing communities and decreasing supplies of fish. “The dam is likely a harbinger of things to come,” NASA says, adding that several dams will be added to the Mekong in the future.

Bolten says it’s crucial to think of rivers as shared resources. “And not just with our friends and neighbours, but with future generations,” he said. “I’m always thinking about what my daughter’s reality with water is going to be.”